The present invention relates to a method for preparing a polyester resin copolymerized with 1,4-cyclohexanedimethanol. More specifically, the present invention relates to a method for preparing a polyester resin, in which polycondensation in the presence of a titanium-based compound as a catalyst and a carboxy phosphonic acid typed compound as a stabilizer yields a polyester resin copolymerized with 1,4-cyclohexanedimethanol, which is of high clarity with good color.
Recently, polyester resins copolymerized with 1,4-cyclohexanedimethanol have been increasing in commercial value, finding various applications, including packaging materials, molded articles, films, etc. In the past, such polyester resins were prepared by the addition of 1,4-cyclohexandimethanol upon the transesterification of dimethylterephthalate. However, the polyester is now produced from terephthalic acid; thus bringing about an improvement in the quality of the final product and an economic benefit. Such a technique is explained in U.S. Pat. Nos. 5,340,907 and 5,385,773. These patents disclose a process for producing copolyester having a dicarboxylic acid component and a glycol component, the dicarboxylic acid component comprising repeat units from 90 mol % terephthalic acid and the glycol component comprising repeat units from about 10 to 95 mol % 1,4-cyclohexanedimethanol and from about 90 to 5 mol % ethylene glycol.
Leading to the present invention, the intensive and thorough research on polyester resins, carried out by the present inventors aiming to avoid the problems encountered in prior arts, resulted in the finding that polycondensation in the presence of a titanium-based compound as a catalyst and carboxy phosphonic acid typed compound as a stabilizer yields a polyester resin copolymerized with 1,4-cyclohexanedimethanol, which is of excellent clarity with good color.
Accordingly, it is an object of the present invention to provide a method for preparing a polyester resin copolymerized with 1,4-cyclohexanedimethanol having improved clarity and color. The polyester resin of the present invention has a brighter color and lower metal content than those of a conventional polyester resin.
To achieve the object of the present invention, the method for preparing the polyester resin copolymerized with 1,4-cyclohexanedimethanol comprises the following steps of: esterifying terephthalic acid with a glycol component comprising ethylene glycol and 1,4-cyclohexanedimethanol at 230 to 270xc2x0 C. under pressures of 0.1 to 3.0 kg/cm2 with the molar ratio of the whole glycol component to terephthalic acid ranging from 1.1 to 3.0; and polycondensing said esterified reaction product at 250 to 290xc2x0 C. under a reduced pressure of 400 to 0.1 mmHg in the presence of titanium-based compound as a polycondensation catalyst and carboxy phosphonic acid typed compound represented by the following formula I as a stabilizer: 
wherein R1, R2 and R3, which are the same or different, each being selected from the group consisting of hydrogen, alkyl groups having 1 to 10 carbon atoms, cycloalkyl groups having 1 to 10 carbon atoms and aryl groups having 6 to 10 carbon atoms; and R is selected from the group consisting of alkylene groups having 1 to 10 carbon atoms, cycloalkylene groups having 1 to 10 carbon atoms, and arylene groups having 6 to 10 carbon; atoms.
In preparing a polyester resin copolymerized with 1,4-cyclohexanedimethanol using terephthalic acid, ethylene glycol and 1,4-cyclohexanedimethanol, the glycol component comprising ethylene glycol and 1,4-cyclohexanedimethanol are primarily fed with the molar ratio of the whole glycol component to terephthalic acid ranging from 1.1 to 3.0 and then esterified at 230 to 270xc2x0 C. under a pressure of 0.1 to 3.0 kg/cm2. Available 1,4-cyclohexanedimethanol in the present invention may be in the form of a cis- or a trans-isomer or may be a mixture of both isomers. 1,4-cyclohexanedimethanol is added at an amount approximated to a desired mole % of the final polymer. In the present invention, 1,4-cyclohexanedimethanol amounts to 10 to 90 mole % of the whole glycol component so as to prevent the difficulties in polymer processing processes due to crystallization.
As for the esterification, it is preferably carried out at 240 to 270xc2x0 C. and more preferably at 245 to 260xc2x0 C. Meanwhile, the esterification time generally varies from 100 to 300 minutes, depending on reaction temperature, pressure, molar ratio of glycol to terephthalic acid. When a preparation method of polyester is divided into an esterification step and a polycondensation step as in the present invention, no catalysts are needed in the esterification.
The esterification can be carried out in a batch or continuous process. The raw materials can be separately fed, but the most preferable feeding form is a slurry of terephthalic acid in glycol.
After the esterification step is completed, the polycondensation step is carried out. In advance of the starting of the polycondensation step, a polycondensation catalyst is added to the esterified reaction product, along with a stabilizer and a color toning agent.
Generally, the polycondensation catalyst is selected from the group consisting of titanium, germanium, and antimony compounds. Among these, the titanium-based catalyst is the most advantageous since it is used at a smaller amount than the antimony-based catalyst and is less expensive than the germanium-based catalyst. Examples of the compounds useful as titanium-based catalysts include tetraethyl titanate, acetyltripropyl titanate, tetrapropyl titanate, tetrabutyl titanate, polybutyl titanate, 2-ethylhexyltitanate, octyleneglycol titanate, lactate titanate, triethanolamine titanate, acetylacetonate titanate, ethylacetoacetic ester titanate, isostearyl titanate, titanium dioxide, coprecipitate of titanium dioxide and silicon dioxide, coprecipitate of titanium dioxide and zirconium dioxide. These compounds may be used alone or in combination. The catalyst is used at the amount of 5 to 100 ppm as based on the titanium element in the final polymer. The catalyst has an influence on the color of the final polymer. Other factors affecting the color include the stabilizer and the color toning agent to be used.
Generally, stabilizers used in the polycondensation are exemplified by phosphoric acid and trimethylphosphate, and triethylphosphate. In the present invention, a carboxy phosphonic acid typed compound represented by the following formula I is used as a stabilizer to obtain the best color and reactivity. 
wherein R1, R2 and R3, which are the same or different, each being selected from the group consisting of hydrogen, alkyl groups having 1 to 10 carbon atoms, cycloalkyl groups having 1 to 10 carbon atoms and aryl groups having 6 to 10 carbon atoms; and R is selected from the group consisting of alkylene groups having 1 to 10 carbon atoms, cycloalkylene groups having 1 to 10 carbon atoms, and arylene groups having 6 to 10 carbon atoms.
Representative of the carboxy phosphonic acid typed compounds, triethylphosphonoacetate is more stable in heat than conventional stabilizers, thus minimize the formation of degradation products and residues. Also, the carboxy phosphonic acid typed stabilizer of the present invention shows lower volatility than that of the stabilizers used in the prior art. Additionally, corrosion to equipments and toxicity can be reduced owing to the use of the carboxy phosphonic acid typed stabilizers of the present invention, compared with the conventional stabilizer. The stabilizer of the present invention is used at an amount of 10 to 150 ppm of phosphorus element based on the final polymer and preferably at an amount of 10 to 100 ppm. For example, if the amount of the stabilizer is less than 10 ppm, an insufficient stabilizing effect results, causing the polymer to undergo yellowing. On the other hand, if the amount exceeds 150 ppm, a desired polymerization degree cannot be obtained.
With regard to the color toning agent which is added to improve the color of the polymer, examples thereof include cobalt acetate, cobalt propionate and etc. This agent is added at an amount of 0 to 100 ppm based on the weight of the final polymer. An organic compounds, in addition to those color toning agents, may be used to control the color of the polymer.
At 250 to 290xc2x0 C. under a reduced pressure of 400 to 0.1 mmHg, the polycondensation step is carried out for a period of time required for the polymer to reach a desired inherent viscosity. A preferable reaction temperature for the polycondensation is within the range of 265 to 280xc2x0 C. The reduced pressure condition is also for the purpose of eliminating glycol. After completion of the polycondensation, a desired polyester resin copolymerized with 1,4-cyclohexanedimethanol can be obtained.